There have been a wide variety of powered knife sharpeners introduced to the market that depend for their performance upon relatively precise control of the sharpening angle. The accuracy of angular control in such devices commonly is inadequate to take full advantage of the edge sharpness that can be achieved with ultrafine abrasives.
The ultimate precision of electric sharpeners that use abrasives to create the final knife edge depends critically on the size of the abrasive particles that are used to abrade the final edge and on the precision of all mechanical and structural elements that are directly involved in establishing and maintaining consistently the sharpening angle between the plane of each final edge facet and the plane of the abrasive sharpening surface.
As described in U.S. Pat. No. 6,875,093 as finer, smaller grit, abrasive particles are used in precision sharpeners in order to obtain a smoother hence more polished surface on the facets of the blade being sharpened, it becomes necessary to reduce the pressure applied to the edge facet during sharpening in order to minimize the size of the burr created at the edge and to avoid “loading” of the abrasive surface composed of ultrafine particles. Also to realize the ultimate precision as the moving abrasive surface machines the facet the angular relationship of the plane of the moving abrasive surface at the point of contact with the facet must be held precisely at the same angle throughout each physical stroke or repetitive motion of the abrasive surface. If the active abrasive surface is in the form of rotating circular structure such as a disk, FIG. 1, the spacial precision of the moving circular line of contact between the facet and the disk surface places a limit on the consistency of the sharpening angle. If the spacial precision is high then the sharpening angle will remain very consistent during each revolution of the line of contact between the facet surface and the moving abrasive surface. Higher angular precision results in sharper edges on the knives.
U.S. Pat. No. 6,875,093 emphasizes that when springs such as spring 5 (FIG. 1) of lower force constant are used to reduce the pressure during sharpening with a disk 2 covered with ultrafine abrasives, any small mechanical imperfections in the surface of rotation will cause serious vibrations, intermittent contact with the facet and variations in the sharpening angle as the contacting abrasive surface goes through each cycle of its motion.
If the abrasive surface is established on the surface of a rotating disk-like surface, (FIG. 1) any runout (wobble) of that surface, about its axis of disk rotation, and any micro or macro imperfections in that disk-like surface of rotation can cause significant changes in the sharpening angle during each rotation cycle. Such angular changes deteriorate the precision with which the facet surface is machined. Changes in the sharpening angle on each cycle limit the precision with which the edge (intersection line of the two facets) is formed and hence establish the obtainable sharpness of the edge and the size of burr that is created along that edge. With a more consistent sharpening angle, the residual burr will be smaller and the smaller the residual burr the sharper the knife edge will be.